Water-filtered infrared-A (wIRA) in acute and chronic wounds
Water-filtered infrared-A (wIRA), as a special form of heat radiation with a high tissue penetration and a low thermal load to the skin surface, can improve the healing of acute and chronic wounds both by thermal and thermic as well as by non-thermal and non-thermic effects. wIRA increases tissue temperature (+2.7°C at a tissue depth of 2 cm), tissue oxygen partial pressure (+32% at a tissue depth of 2 cm) and tissue perfusion. These three factors are decisive for a sufficient supply of tissue with energy and oxygen and consequently also for wound healing and infection defense.
wIRA can considerably alleviate pain (without any exception during 230 irradiations) with substantially less need for analgesics (52–69%less in the groups with wIRA compared to the control groups). It also diminishes exudation and inflammation and can show positive immuno-modulatory effects. The overall evaluation of the effect of irradiation as well as the wound healing and the cosmetic result (assessed on visual analogue scales) were markedly better in the group with wIRA compared to the control group. wIRA can advance wound healing (median reduction of wound size of 90% in severely burned children already after 9 days in the group with wIRA compared to 13 days in the control group; on average 18 versus 42 days until complete wound closure in chronic venous stasis ulcers) or improve an impaired wound healing (reaching wound closure and normalization of the thermographic image in other-wise recalcitrant chronic venous stasis ulcers) both in acute and in chronic wounds including infected wounds. After major abdominal sur-gery there was a trend in favor of the wIRA group to a lower rate of total wound infections (7% versus 15%) including late infections following discharge from hospital (0% versus 8%) and a trend towards a shorter postoperative hospital stay (9 versus 11 days).
Even the normal wound healing process can be improved.
The mentioned effects have been proven in six prospective studies, with most of the effects having an evidence level of Ia/Ib.
wIRA represents a valuable therapy option and can generally be recom-mended for use in the treatment of acute as well as of chronic wounds.
The application of water-filtered infrared-A (wIRA) for the improvement of healing of acute and chronic wounds and the underlying principles are described more extensively than here in the three reviews , , , which belong together (in total 42 PDF pages). Please refer to these reviews for more details and references. Besides this, two further reviews concerning this subject ,  and one review on a slightly broader subject  are available.
Working mechanisms of wIRA
The experience of the pleasant heat of the sun in moder-ate climatic zones arises from the filtering of the heat radiation of the sun by water vapor in the Earth’s atmo-sphere , , , , , , see Figure 1. The filter effect of water decreases those parts of infrared radiation (most parts of infrared-B and -C and the absorption bands of water within infrared-A), which would otherwise – by reacting with water molecules in the skin – cause an un-desired thermal load to the surface of the skin , ,, , , . Technically, water-filtered infrared-A (wIRA) is produced by special radiators, whose full spec-trum of radiation of a halogen bulb is passed through a cuvette containing water, which absorbs or decreases the described undesired wavelengths of the infrared ra-diation , , see Figure 2. Within the infrared range, the remaining wIRA (within 780–1400 nm) mainly con-sists of radiation with good tissue penetration properties and therefore allows – compared to unfiltered heat radi-ation – a multiplication of the energy transfer into tissue without irritating the skin, similar to the sun’s heat radi-ation in moderate climatic zones. Typical wIRA radiators emit no ultraviolet (UV) radiation and almost no infrared-B and -C radiation and the amount of infrared-A radiation in relation to the amount of visible light (380–780 nm) is accentuated , , see Figure 3.
Within the spectra of infrared-A and water-filtered infra-red-A, radiation effects in particular of the energy-rich wavelengths near to visible light – approximately 780–1000 nm (800–900 nm , , , 800 nm , 820 nm , , , 830 nm ) – have been described both in vitro and in vivo. These wavelengths seem to represent the clinically most important part of the infrared-A and wIRA range , .
Water-filtered infrared-A as a special form of heat radi-ation with a high tissue penetration and with a low thermal load to the skin surface (see Figure 4), acts both by thermal (related to heat energy transfer) and thermic (temperature dependent, with a relevant change of tem-perature) as well as by non-thermal (without a relevant transfer of heat energy) and non-thermic (not depending on temperature, without a relevant change of tempera-ture) effects . wIRA produces a therapeutically usable field of heat in the tissue and increases tissue tempera-ture , , , , , , , , tissue oxygen partial pressure , and tissue perfusion ,, , . These three factors are vital for a suffi-cient supply of tissue with energy and oxygen.
As wound healing and infection defense (e.g. granulocyte function including its antibacterial oxygen radical forma-tion) depend decisively on a sufficient supply of tissue with energy and oxygen and since the centers of chronic wounds are often relatively hypothermic , , (while e.g. both preoperative  and postoperative , heat supply to the operation field can improve heal-ing of acute wounds) and frequently have an oxygen partial pressure close to zero , , , , ,, , , , , , , , one explanation for the good clinical effect of wIRA on wounds and wound infections could be the improvement of both the energy supply per time (increase of metabolic rate) and the oxygen supply . In addition, wIRA has non-thermal and non-thermic effects, which are based on a direct stimulation of cells and cellular structures: Reactions of cells to infrared radiation – partly found even at very small irradiances – are e.g. target-oriented growth of surface extensions (plasmodia) , influence on cytochrome c oxidase , , , target-oriented growth of neurons , stimulation of wound repair ,  as well as cell protective effects of infrared-A , , ,  and water-filtered infrared-A (wIRA) , , .
wIRA can considerably alleviate pain (with remarkably less need for analgesics) and diminish an elevated wound exudation and inflammation and can show positive im-munomodulatory effects. wIRA can advance wound healing or improve an impaired wound healing both in acute and in chronic wounds, including infected wounds. Even the normal wound healing process can be improved , .
wIRA is contact-free, easily applied, involves no discomfort to the patient or the use of expendable materials and is effective even in deeper-lying tissue regions. wIRA appli-cation, with appropriate therapeutic irradiances and doses, could be shown not only to be harmless for human skin , , , , , , but even to have pro-tective effects in cells against damage caused by UV ra-diation , , , , , , , , . Safety aspects of the clinical use of wIRA have been de-scribed extensively, especially in  and . Particularly when  and the current review  are taken into consideration, the application of wIRA with adequate irra-diances can be considered as being safe. The irradiation of the typically uncovered wound is carried out using a wIRA radiator, see Figure 5.
Clinical effects of wIRA on wounds
Based on 6 clinical studies, the following has been proven with a level of evidence of Ia/Ib , :
* acute pain reduction during wIRA irradiation
* reduction of the required dose of analgesics
* faster reduction of wound area
* better assessment of wound healing
* better overall evaluation of the effects of irradiation (including pain, wound healing, cosmesis)
* higher tissue oxygen partial pressure during wIRA
* higher subcutaneous temperature during wIRA
* better cosmesis
In addition, the following trends have been found:
* lower rate of wound infections
* shorter postoperative hospital stay
Additional clinical observations are:
* reduction of inflammation
* reduction of hypersecretion
Figure 1: Spectral solar irradiance outside the atmosphere and on the surface of the Earth at sea level,
in both cases with the sun at the zenith and for a mean Earth-sun distance. Shaded areas indicate absorption before reaching the surface of the Earth at sea level due to the atmospheric constituents shown (from , , adapted from ).
For comparison of Figures 1 and 3: 1000 W · m–2 · µm–1 = 100 mW · cm–2 · µm–1 = 1 mW · cm–2 · (10 nm)–1
Figure 2: Cross-section of a water-filtered infrared-A radiator (Hydrosun, Müllheim, Germany)
The whole incoherent non-polarized broadband radiation of a 3000 Kelvin halogen bulb is passed through a cuvette containing water, which absorbs or decreases the undesired wavelengths within the infrared region (most parts of infrared-B and -C and the absorption bands of water within infrared-A). The water is hermetically sealed within the cuvette. A fan provides air cooling of the cuvette to prevent the water from boiling. (from )
Figure 3: Comparison of the spectra of the sun on the surface of the Earth at sea level and of a water-filtered infrared-A radiator Spectral solar irradiance on the surface of the Earth at sea level (with the sun at the zenith and for a mean Earth-sun distance) as in Fig. 1 (from , adapted from ) and spectral irradiance of a water-filtered infrared-A radiator (Hydrosun® radiator 501 with 10 mm water cuvette and orange filter OG590) at approximately 210 mW/cm² (= 2.1 · 10³ W/m²) total irradiance
(from , ).
The spectrum of the sun at sea level includes ultraviolet radiation (UV, <400 nm), visible light (VIS, 380–780 nm), and infrared radiation (IR, >780 nm). The spectrum of the water-filtered infrared-A radiator includes only visible light (VIS) and infrared radiation (IR); the visible part depends on the color filter used; the wIRA radiator does not emit ultraviolet radiation (UV).
Both spectra show the decreased irradiances of the absorption bands of water.
Figure 4: Comparison of irradiation with water-filtered infrared-A and with conventional infrared
Thermographical comparison of skin surface temperatures in the lumbar region 12 minutes after commencement of irradiation with water-filtered infrared-A (left) and conventional infrared (right) with the same irradiance: the skin surface temperature is higher in the case of irradiation with conventional infrared (presented in the thermography), while the temperature at 1 cm tissue depth is higher when irradiating with water-filtered infrared-A (from , ). Water-filtered infrared-A thus leads to a high tissue penetration combined with a low thermal load to the skin surface.
Figure 5: Example of an irradiation of a wound with a water-filtered infrared-A radiator
(published with kind approval of Prof. James Mercer, Tromsø/Norway) (from , )